UCP1-deficiency causes brown fat respiratory chain depletion and sensitizes mitochondria to calcium overload-induced dysfunction
UCP1 is a key feature of thermogenic
fat cells, both brown and beige. We have demonstrated here that upon cold
exposure interscapular BAT of UCP1-KO mice has global mitochondrial disruptions,
which extend well beyond the deletion of UCP1 itself. These data reveal physiological
interactions between UCP1 and ROS. The role of UCP1 itself in the regulation of
ROS production is not fully understood. Evidence in support of a robust role
for UCP1-mediated uncoupling in the regulation ROS production in vitro has been provided (6, 7, 41), while findings suggesting a limited role for UCP1
activity in controlling ROS in vitro
has also been presented (8, 42-44). Importantly, UCP1 does seem to play a role in regulating
BAT redox tone in vivo (9), and acute adrenergic
stimulation in vivo drives ROS
production to support UCP1-dependent thermogenesis (10).
Our findings demonstrate that UCP1-deficient
BAT mitochondria are poorly equipped at buffering calcium in a ROS-dependent
manner. Most importantly, we show that the acquired molecular and functional differences
between BAT mitochondria from WT and UCP1-KO animals are more widespread than
the deletion of UCP1 itself. Considering the striking alterations to the BAT
mitochondrial proteome (substantial reduction of ETC abundance) of UCP1-KO
mice, caution must be taken when attributing a BAT phenotype solely to UCP1
deletion in these animals. In addition, reduced ETC expression may be commonly
associated with decreased UCP1 levels more generally, and should be kept in
mind when studying genetic models with reduced BAT UCP1 expression. Notably, our
findings here suggest that the reduced capacity of UCP1-KO BAT to activate
oxidative metabolism following adrenergic administration (cold or chemical) is at
least partly due to reduced expression of the ETC, and not solely due to lack
of UCP1-mediated uncoupling.
Examination
of calcium sensitivity of BAT mitochondria with and without UCP1 adds further
evidence to the relevance of mechanisms of ROS production in BAT. Previous
studies have compared ROS production between WT and UCP1-KO BAT mitochondria
when using G3P as a respiratory substrate, indicating either comparable (8), or enhanced (6, 7) levels. Importantly, G3P-mediated mitochondrial
energization can drive ROS production by RET or from mitochondrial
G3P dehydrogenase (GPD2) itself (7, 30). Moreover,
GPD2 appears to have the capacity to produce ROS in the mitochondrial IMS (30, 41, 45), as opposed to Complex I that produces superoxide in
the mitochondrial matrix (12).
This compartmentalization of ROS production is a plausible explanation
for the sensitivity of UCP1-KO mitochondria to succinate mediated ROS
production, which drives superoxide production principally through complex I (12). Since G3P-mediated ROS production
can drive ROS independently of Complex 1 (i.e. at GPD2 itself) (7, 30, 41), our data in sum suggest that Complex I-mediated ROS
production by RET is a major contributor to mitochondrial dysfunction in
UCP1-KO BAT. This interpretation is in
line with the recognized importance of ROS originating in the mitochondrial
matrix supporting permeability transition (46, 47). Interestingly, GPD2 abundance was unaltered in
UCP1-KO BAT (Table S1) suggesting that in the absence of UCP1, G3P-mediated
electron flux to coenzyme Q (CoQ) is maintained. Previous investigations have noted quantitatively
different effects of G3P-driven ROS production in BAT mitochondria between WT
and UCP1-KO animals (6-8). In light of our findings, these discrepancies may be
predictive of differential mitochondrial adaptation in different UCP1-KO mice
colonies to mitigate ROS-sensitivity due to genetic absence of UCP1. Such
differences might be expected to arise on congenic (i.e. C57BL/6J and
129/SvImJ) backgrounds, which are particularly sensitive to ablation of UCP1 (48), and may therefore be prone to
selection against enhanced ROS production depending on breeding strategy. More generally, the functional effects that arise
from distinct ROS sites in BAT upon thermogenesis is an interesting avenue of
research to pursue in the future.
The data presented here indicate
that mice genetically lacking UCP1 exhibit a plethora of acquired features that
extend substantially beyond the deletion of UCP1 itself. These defects, such as
the striking reduction of mitochondrial ETC components should be considered
when using this mouse model to study UCP1 function. However, this model may
have utility for examination of general features of mitochondrial dysfunction.
For example, the molecular processes regulating the discordance between ETC
protein and mRNA abundance in cold-exposed UCP1-KO animals may be an
appropriate model for studying fundamental mechanisms of mitochondrial proteostasis.
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